Glossary O

OB ASSOCIATION - Loose association of massive O and B stars formed from a giant molecular cloud surrounded by an HII region. Once the radiation and outflows of these short-lived massive stars have dispersed the gas remaining after star formation, the potential well that holds the association together weakens, and the stars drift apart.

OCCAM'S RAZOR - Principle that all other things being equal, the simplest explanation is to be preferred.

OCTAHEDRITE - Commonest type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of the kamacite crystals. When sliced, etched with a weak acid, and polished, they display a characteristic Widmanstätten pattern. Spaces between larger kamacite and taenite plates are often filled by a fine-grained mixture of kamacite and taenite called plessite. Octahedrites are divided into several groups based on the width of their kamacite lamellae, and each subgroup is associated with a particular chemical class of iron meteorites.

OCTET RULE - Atoms tend to gain or lose electrons in order to attain an electron configuration resembling a noble gas. Since the outermost shell in these cases is comprised of an s orbital and three p orbitals, the maximum number of electrons in the valence shell is eight (the octet). Some common elements which do not follow the octet rule are: H (needs only 2 electrons to complete its shell) and B (only needs six electrons).

OLDHAMITE - Mn-Ca sulfide, (Mn,Ca)S, found in minor amounts in highly reduced meteorites (aubrites and all E chondrites).

OLIVINE- Group of Fe-Mg silicate minerals with a compositional range running from all Mg (forsterite: Mg₂SiO₄) to all Fe (fayalite: Fe₂SiO₄). Olivine is common in pallasites, chondrites and some chondrites.

OLIVINE-BRONZITE - Obsolete name previously applied to H chondrites, because they are composed of olivine and bronzite (pyroxene with ~20 wt. % FeSiO₃).

OLIVINE-HYPERSTHENE - Obsolete name previously applied to L chondrites, because they are composed of olivine and hypersthene (pyroxene with 22-30 wt. % FeSiO₃).

OMEGA (Ω) - The cosmological density parameter. This has the components of mass density, Ω_M, and vacuum energy density, Ω_L.

OORT CLOUD - Vast spherical swarm ~10¹²-10¹³ long-period comets orbiting the Solar System with semimajor axes between 1,000 and 50,000 AU (0.25 pc), typically with low orbital eccentricity. The Oort cloud, while roughly spherical at the largest radius, is wedge-shaped where it merges with the outer planet region in the vicinity of the Kuiper belt of comets. The cloud is divided into different regions of dynamical stability: the Kuiper Belt (35-50 AU; affected by planetary perturbations), a dynamically inert region (50-2000 AU; not affected by gravity of planets or stars), the inner Oort cloud (2,000-15,000 AU; affected by galactic tidal forces), and the outer Oort cloud (15,000-100,000 AU; affected by stellar perturbations). The Oort cloud has never been observed rather its existence is inferred from analysis of the orbits of comets, which come in from the cloud. Recent discoveries of Kuiper Belt objects have confirmed its existence.

ORDINARY CHONDRITE - A chondrite that is composed mostly of olivine, orthopyroxene, and a certain percentage of more or less oxidized Ni-Fe metal. Based on the differing content of metal and differing mineralogical compositions, the ordinary chondrites are divided into three distinct categories: H Group, L Group, and LL Group. Subtypes of ordinary chondrites are determined using the petrographic criteria given in the table below.

For very low Type 3 subtypes, division may be made using the average concentration and standard deviation of Cr₂O₃ contents in olivine (Grossman & Brearley, 2005).

ORGANIC - Pertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes.

ORGUEIL METEORITE - A large carbonaceous chondrite that disintegrated and fell in fragments near the French town of Orgueil on May 14, 1864. About 20 pieces, totaling ~12 kg in mass, were subsequently recovered from an area of several square km, some head-sized but most were smaller than a fist. Specimens could be cut with a knife and, when sharpened, could be used like a pencil. Pristine pieces of Orgueil contain a relatively simple mixture of amino acids, consisting primarily of glycine and β-alanine. The carbon isotope composition of the amino acids indicate that they do not represent terrestrial contamination, but formed in space. Whereas CM carbonaceous chondrites contain a complex mix of amino acids made up of more than 70 different types of amino acids, Orgueil (and Ivuna) seem to be made up primarily of just two amino acids. The CM meteorites are widely believed to be pieces of an asteroid, but the amino acid signatures within Orgueil are consistent with formation from components such as hydrogen cyanide, which have been recently observed in the comets Hale-Bopp and Hyakutake. This suggests that the organic material in Orgueil and Ivuna is the product of reactions that once took place in the nucleus of a comet, which, if true, would make these meteorites the first to be identified as having come from a comet nucleus.

ORIENTED METEORITE - Meteorite with a conical shape, which results when a meteorite has a stable flight through Earth's atmosphere and undergoes uniform ablation. The leading side and sides often have flow-lines in the surface which indicate the direction the material was being ablated. The rear edges of the trailing side can create a "roll-over" rim or "lip" where the ablating/melting material has started to flow over onto the back side.

ORTHOPYROXENE - Orthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO₃ - to Fe-rich ferrosilite, FeSiO₃.

ORTHOPYROXENITE (OPX) - A type of Mars meteorite that is currently represented by only one known specimen, ALH 84001, which was found in the Allan Hills region of Antarctica in 1984. It is a cumulate rock consisting of 97% coarse-grained, Mg-rich orthopyroxene, with small amounts of plagioclase, chromite, and carbonate. It was initially classified as a member of the HED group, specifically a diogenite, because these achondrites are also made mostly of orthopyroxene. However, the presence of oxidized Fe in the chromite of ALH 84001 led to its reclassification as a Martian meteorite; a fact that has been subsequently confirmed by its oxygen isotope composition. With a crystallization age of ~4.4 Ga, it is by far the oldest meteorite from Mars yet discovered. It probably represents a sample of the early crust, providing evidence for the earliest geologic history of Mars.

OUT-OF-PHASE - Situation with waves when the peaks from one wave are exactly matched by the troughs of a second wave. If these two waves collide and have the same amplitude, they will mutually cancel. That is, for every positive point along one wave, the second wave would have a negative counterpart of the same magnitude. When these points are added together, they cancel leaving no net amplitude. This kind of phenomena is often called destructive interference.

OXIDATION - Chemical reaction in which electrons are formally transferred between atoms in the reactants and products. When iron rusts it is undergoing an oxidation reaction.

OXYGEN (O) - Element that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only ~1/1500 that of H. Atmospheric oxygen normally occurs in molecular form, O₂. Ozone, O₃, and monatomic oxygen, O^2-, predominate in the upper atmosphere, where ozone shields the Earth from the Sun's UV radiation. Oxygen combines with most other elements and, when this reaction occurs rapidly (with release of heat and light), it is described as combustion, or burning. Oxygen usually occurs bonded with another element (e.g., CO, CO₂), being too reactive to persist very long as O₂. All O₂ in Earth's atmosphere is constantly renewed by photosynthetic activity. Oxygen is very common as ionic crystalline solids bonded with various metal ions to yield "oxides" and bonded with Si to form silicates.

Oxygen has three stable isotopes: ¹⁶O (99.757%), ¹⁷O (0.038 %), and ¹⁸O (0.205 %). Variations in the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios are reported in terms of their deviations (δ¹⁸O and δ¹⁷O) in parts per thousand (‰) from a standard. The usual reference is standard mean ocean water (SMOW), which plots at (0,0). Pure ¹⁶O plots at -1000 ‰ on both axes.

The oxygen atoms in terrestrial (and lunar) rocks do not have identical proportions of the three isotopes, but the ratios of these isotopes follow a simple relationship controlled by their masses. Rocks with the same ¹⁸O/¹⁶O ratio will have the same ¹⁷O/¹⁶O ratios. But if their ¹⁸O/¹⁶O ratios differ by say 0.2 %, their ¹⁷O/¹⁶O ratios will differ by half this amount, 0.1 %. This behavior, called "mass-dependent fractionation," yields the "terrestrial mass-fractionation line" (slope = 0.52) on the isotope plot. All terrestrial rocks plot along this line, as do materials from the Moon. This coincidence of Earth and Moon isotopes is taken as evidence that the two bodies share a common origin. The mass-fractionation lines for other planets (Mars, 4 Vesta) are parallel, indicating slightly different rations of the three isotopes, but still mass-dependent fractionation. In contrast, meteorites show evidence of formation in many isotopically distinct environments, permitting classification based on oxygen isotopic compositions.

OXYGEN FUGACITY - Measure of the availability of oxygen to react.